The present invention relates to a rotationally symmetrical anchoring element (implant) for anchoring in bone tissue, for example for permanent anchoring of artificial teeth and dental bridges in the jaw bone. The anchoring element is made of a biocompatible material, for example titanium, and is intended for incorporation with bone tissue. Following implantation, the upper portion of the implant is located outside the bone tissue and is designed for the purpose of attachment thereto of a spacer piece or prosthetic structure.
It is already known to permanently anchor oral and extraoral prostheses in bone tissue with the aid of bone-anchoring elements made of titanium. To avoid loosening of the prosthesis, it is necessary to provide a healing-in period with direct contact, i.e. an exact match without intermediate connective tissue, between the anchoring element and the surrounding bone tissue. Such a direct contact between the bone-anchoring element and the surrounding bone tissue has been found to provided the best conditions for permanent anchoring of, for example, a tooth prosthesis.
The anchoring elements, which are in most cases screw-shaped, are surgically inserted into the jaw bone, specially prepared for this purpose, in a two-stage procedure, or the operation is performed in one stage. In the two-stage procedure, which has so far been the most suitable for jaw bone operations, the anchoring element is surgically inserted into the bone in a first operation. This is followed by an adequate healing-in period during which the upper end surface of the anchoring element is completely covered by intact mucous membrane. During the healing phase, the bone tissue grows firmly onto, and forms one unit with the implanted anchoring element. In a second operation the anchoring element is exposed, by having the mucous membrane surgically punctured, and an extension piece or spacer piece is attached to the anchoring element. When the operation is performed in one stage, the anchoring element is already initially allowed to penetrate the mucous membrane, and then the attachment to the spacer piece can be carried out, after a suitable healing-in period, without blood loss (without surgical intervention).
The spacer piece is generally attached to the anchoring element by means of a spacer screw which is screwed into a central, internally threaded bore in the anchoring element. Alternatively, the spacer piece can be cemented to the anchoring element via a screw or pin which runs down into the bore.
However, such a bore in the central attachment part of the anchoring element is a limiting factor in terms of its implantation. The internal bore represents a production step which increases costs, because the thread has to be cut to small dimensions with high precision.
In addition, the central bore inevitably entails a material reduction in the loaded part of the anchoring element. As a result there is an increased risk of fracturing unless this is compensated for in an appropriate way, generally by making the anchoring element thicker than would otherwise be the case. The anchoring element is thus given a minimum diameter below which, for reasons relating to strength, it is not possible to go, because of the forces to which the element is exposed in, for example, the jaw bone during mastication.
It has also been proposed to attach a prosthetic structure directly to the anchoring element without any intermediate spacer piece, for example in Swedish Patent Application 95.03291-8 of Dan Lundgren. One advantage of such a solution is that it requires one component less. Also, the central bore in the upper part of the anchoring element can be made with a smaller diameter, which permits introduction of slightly narrower anchoring elements without any risk of fracturing on account of the fact that the material thickness is too small in the wall between the central bore and the circumferential surface of the anchoring element. However, even though the bore can be made with a smaller diameter, it is still necessary, even in this case, to have a bore which reduces the material thickness and therefore inevitably limits the smallest critical diameter of the anchoring element.
There are many reasons why it is desirable to use narrower anchoring elements. Such elements can be used in bone areas where the available bone width is much smaller than before. There are a number of such applications in which the available bone width has been too small to allow present-day implants to be used in a clinically reliable manner because these implants have been too thick.
It is already known to design a bone-anchoring element without an upper bore and with a nut-like sleeve applied to the threaded circumferential surface, see U.S. Pat. No. 4,122,605 of Hirabayashi et al. The sleeve n is threaded onto the anchoring element to the desired position so as to bear against the bottom surface of a bore 6 in the bone surface b1. The prosthetic structure in the form of a tooth is then connected to the sleeve by means of cement 4. The aim of this arrangement is quite different, however, namely to provide a counter force to the bone in order to increase stability.
Although such a construction should in theory make it possible to use narrower fixtures, its practical application is limited. If the nut-like sleeve were to function as the spacer piece, then this would not have any exact position in relation to the screw-shaped circumferential surface, with all the disadvantages that this entails, and the open, nut-like sleeve obviously only permits cemented solutions.
One object of the present invention is to make available an anchoring element which simplifies the implantation, for example reduces the number of components required, and which affords advantages in terms of production engineering, and in which the attachment between anchoring element and spacer piece can be formed by a screw connection, with the flexibility which characterizes such a connection.
Another object of the invention is to make available a tighter connection in order to reduce the risk of bacterial invasion and inflammatory infiltrates in the soft tissue which surrounds the joint between the two implant parts.
Yet another object of the invention is to make available an anchoring element which can be made to heal into bone areas of much smaller bone width than has previously been possible, without any risk of loosening or fracturing of the anchoring element, and which element dose not have the disadvantages discussed above in connection with Hirabayashi et al.
According to the invention, this is achieved by means of the fact that the anchoring element has the features that include the upper portion of the circumferential surface, designed for attachment of the spacer piece or of the prosthetic structure, and including a smooth (unthreaded) conical portion whose diameter increases in the direction away from the upper end surface (gable surface) of the element, this conical portion forming a bearing surface for the spacer piece or the prosthetic structure.
An anchoring element designed in this way affords several advantages:
The anchoring element is easier to implant since it includes one component less for the spacer attachment.
The anchoring element is simple to produce.
The anchoring element can be implanted in jaws with dental crests of very small (buccolingual) bone width.
The anchoring elements can be placed tighter together in the mediodorsal direction of the dental crest than has been possible with traditional anchoring elements.
The anchoring elements can often be placed between the mandibular arch and the lingual, and in some cases also the buccal, bone surface of the lower jaw.
The positioning between mandibular arch and buccal/lingual bone surface means that two anchoring elements can sometimes be placed in the same buccolingual cross-section instead of one. This provides, overall, the same stability as in the case of one thicker anchoring element in the same region.
The positioning of the narrower implants means that these, to a greater extent than the thicker ones, can be placed in compact bone instead of in cancellous (high-mesh) bone, which increases their contact surface with the surrounding bone.
The small diameter of the anchoring elements means that they can be placed at several locations where there is limited lateral bone space.
The anchoring elements can also be placed in the tops of the interalveolar bone septa during or after extraction of teeth, so that they can be used as temporary or permanent direct implants.
The surgical technique is simplified because it is possible to use a one-stage technique and cutting of edges can be dispensed with.
There is no need for lifting large flaps, which increases patient comfort, saves the jaw bone and makes the intervention easier.
The joint between the anchoring element and the spacer piece or prosthetic structure can be made hermetically tight, or can be omitted altogether, in the alternative embodiments in which implant and spacer piece constitute one continuous piece. This eliminates the risk of bacterial invasion and inflammatory infiltrates in the soft tissue surrounding the joint.
Because the spacer piece can be adapted in terms of its height to the thickness of the soft tissue, an optimum aesthetic appearance can be obtained.
Because the anchoring element can have a small diameter and thus requires very little space, it is also well suited for use in orthodontic treatment (correction of the teeth), i.e. step by step adjustment of the teeth using relatively small forces.